1. Field of the Invention
The present invention relates to a relatively movable unit adapted for use in a linear motion guide unit, a ball nut and screw assembly or the like incorporated in machine tools, assembly machine, testing instruments or the like, and constructed so as to make relatively sliding movement by means of rolling elements which are movable between the confronting raceway grooves.
2. Description of the Prior Art
The recently remarkable development in mechatronics technology requires linear motion guide units improved in precision, speed and miniature and the use of such apparatus extends into many technical fields. The relatively movable units such as linear motion guide units, ball nut and screw assembly or the like are in general employed in various machines and apparatus. The relatively movable units are required more and more to deal with the high-speed moving, miniaturization and increase in load carrying capacity, and also help ensure the long-term durability, low-noise and low-vibration in operation, and determine precisely the position of instruments and devices.
The linear motion guide unit shown in FIG. 9 is a well-known example of a relatively movable unit. The conventional linear motion guide unit has a sliding element, or slider 1, saddling on a track rail 2 so as to freely move along and with respect to the track rail by virtue of circulation of rolling elements, or balls 7, along the raceway grooves 4 on the rail 2. The track rail 2 has on lengthwise side surfaces 3 thereof with raceway grooves 4 and has on lengthwise upper surface thereof with openings 13 spaced from each other. The track rail 2 is fixed together with the mounting base 15, such as a bed, machine base, work table or the like, by screwing bolts through the openings 13 of the track rail 2 and openings in the mounting base 15. The slider 1 has a casing 5 movable with respect to the track rail 2, and end caps 6 bolted at 16 to the opposing ends of the casing 5. Provided on the upper surface of the casing 5 are openings 19 for fixture to other appliances, parts, chucks, grasping jaws or the like. Both of casing 5 and the end caps 6 are formed on the lower surfaces thereof with recesses 10 by which the casing 5 and end caps 6 may saddle on track rail 2 for free movement. The recesses 10 are each formed with a raceway groove 9 in opposition to any one of the raceway grooves 4 on the track rail 2. The rolling elements, or balls 7, are confined for free movement in raceways 26 defined by the raceway grooves 4 and 9. Retainer bands 18 are provided in the casing 5 so as to surround balls 7 to thereby prevent the balls 7 from falling out of the casing 5. Bottom seals 8 are secured to the lower surfaces of the slider 1 for sealing between the track rail 2 and the slider 1.
The end caps 6 are provided with claws for scooping up the balls 7 from the raceway grooves 4 forming the load areas with the track rail 2. The end caps further have turnaround passages for endless circulation of the balls 7. Mounted on the end caps 6 are end seals 17 for keeping sealing function between the track rail 2 and the lengthwise opposing ends of slider 2. The balls 7 move along the load areas, that is, the raceway grooves 4 of the track rail 2 and then come in the turnaround passages in the end caps 6. The balls 7 further move to the return passages 12 which are formed in the casing 5 in parallel with the raceway grooves 9. It will be thus understood that the balls 27 may run in a circulating manner though the raceways, turnaround passages and return passages 12. As a result, the slider 1 may move smoothly and relatively to the track rail 2 by the action of the balls 27 that run through the load areas along the raceways 26. The substance most frequently used as lubricants for the prior linear motion guide unit have been greases or lubricating oils. Greases are fed from grease nipples 11 on balls 7 in the raceways 26, whereas lubricating oils are fed through pipe joints instead of the grease nipples.
Shown in FIG. 10 is a prior ball nut and screw assembly 40 of conventional tube type. The ball nut and screw assembly 40 is comprised of a screw shaft 41 having an external helical groove 43 extending along the outer peripheral surface thereof, and a nut 42 mounted on the screw shaft 41 for translational motion. A tube 45 is secured in the nut 42 by means of a tube retainer 46 and the rolling elements, or steel balls 44, are loaded so as to run in a circulating manner through an internal helical groove 47 in the nut 42 to the tube 45. As any one of the screw shaft 41 and nut 42 rotates with respect to the other, the balls 44 run along the helical groove in the nut 42 whereby any one of the shaft 41 and nut 42 moves relatively to the other in a translational moving relation. A helical raceway 49 on which the balls 44 run is defined by the external helical groove 43 on the screw shaft 41 as a first raceway groove and the internal helical groove 47 in the nut 42 as a second raceway groove. The tube 45 is connected with both of opposing ends of the helical raceway 49 to thereby provide a return passage 48 for endless circulation of the balls 44.
As will be seen from the foregoing, in the prior linear motion rolling guide unit, the balls 7 may run in the slider 1 in a circulating manner as the slider 1 moves linearly along the track rail 2. In the prior ball nut and screw assembly shown, the balls 44 may run in the nut 42 in a circulating manner as the nut 42 moves relatively to the shaft 41.
When the balls 7 run through the load areas defined between the confronting slider 1 and tack rail 2 in the linear motion guide unit, metal-to-metal contact may happen between any adjacent balls. In other words, the balls are brought into the metal-to-metal contact between any adjacent balls closely approaching each other, which closest approach may be caused by the deformation occurring in the raceways owing to the circularity of the ball, the flatness of the track rail, or the uneven load exerted externally. All balls roll in the same direction and thus the peripheral velocities at the contact interfaces of the adjoining balls are reverse in direction with respect to each other. As a result, the frictionally resistant force occurring at the metal-to-metal contact surfaces of the adjoining balls impedes the smooth rolling of the balls. Upon the slider moving in a sliding manner under the condition as described above, the balls primarily roll, but sliding and/or repeated collisions with the adjoining balls or raceway grooves occur, resulting in generation of noise and vibration of relatively high frequency and in premature wear. Hence, the prior linear motion guide unit is apt to be in general reduced in accurate guide ability as well as in durability. In case the moving stroke of the slider 1 along the track rail 2 is less than a few millimeters, it has been experienced that the frictional wear due to a failure of lubricant film is apt to happen at the surfaces of the balls and raceways. However, it has been very hard to eliminate the frictional wear in the prior lubrication systems by which greases or lubricating plates are not applied directly to the balls, but solely applied to the raceways of the track rail.
The prior ball nut and screw assembly has the problem, similar that in the linear motion guide unit, of the frictional wear caused by the metal-to-metal contact of the balls 44.
Any of the linear motion guide units and the ball nut and screw assembly have been recently more and more employed in various wide operating environments including severe lubricating conditions where it is hard to supply the lubricants such as greases, or a scatter of lubricants is not permitted. Further depending upon the operating conditions, the maintenance-free designs may be required, with accompanying reduced lubricant replenishment. To cope with this requirement, it has been already known to incorporate a lubricant-containing member in to the slider.
Disclosed in Japanese Patent Laid-Open No. 170641/1996 is, for example, a linear motion guide unit of self-lubricating type capable of automatically supplying lubricants to the balls. Ball spacers of polymers containing lubricants are each inserted between any adjoining rolling elements and the lubricants exude little by little therefrom to be supplied automatically onto the surfaces of the balls and endless circulating passages.
Japanese Patent Laid-Open No. 54844/1995 discloses another linear motion guide unit of self-lubricating type capable of automatically supplying lubricants to the balls. Polymers containing therein lubricants are used for tubes fitted in holes for ball-return passages in a slider, a part of turnaround paths and ball retainers in raceways defining load areas.
In a linear motion ball bearing unit for guiding linearly a slider body along a rail, disclosed in Japanese Utility Model Laid-Open No. 41724/1989, a slider body is provided with bores for ball-return paths which are communicated for the circulation of balls with raceways defined between the confronting slider body and rail. The bore surfaces are coated with self-lubricating members.
Another type of the linear motion ball bearings, disclosed in Japanese Utility Model Laid-Open No. 53521/1989, is provided with endless circulation paths which are comprised of ball-return zones in a slider body and load zones defined by confronting grooves on the slider body and rail. The circulation paths are filled with balls of solid steel balls and lubricating balls, which are arranged alternately in the paths. The lubricating balls are made of a substance having self-lubricating property or a porous member impregnated with lubricants.
Japanese Patent Laid-Open No. 163550/1990 has for its object to prevent metal-to-metal contact of steel balls with a failure of lubricants. The disclosed ball nut and screw assembly is provided with balls of diameters less than that of the steel balls. The balls are made of materials having self-lubricating property, for example, materials having a layer lattice structure represented by molybdenum disulfide, soft metals represented by gold, silver and lead, and polymers represented by tetrafluoroethylene. The materials exemplified above are solid lubricants that do not vaporize even in the vacuum atmosphere, but may lubricate the interfaces of metals resulting in reducing frictional wear.
According to the prior linear motion guide units of the types described above, at least any one of the raceways and balls is partially composed of materials impregnated with lubricants, so that any special processing should be required to any one or both of the raceways and balls. Such raceways and balls may be automatically fed with lubricants, but they are lacking in mechanical strength withstanding large load because of the lubricant-impregnated materials having no strength as high as the steel.